The use of antibodies as carriers for toxic agents to kill tumor cells selectively has depended upon the coordination of research in three distinct areas: (a) the development of polyclonal or monoclonal antibodies (and their fragments) with specificity for a defined population of cells, such as tumor cells; (b) the elucidation of the chemistry of toxic molecules and the conditions appropriate for their linkage to antibodies; and (c) the production and isolation of naturally occurring toxic molecules. Conjugates of monoclonal antibodies with drugs, plant toxins, and ribosomal inactivating proteins have been summarized by Morgan and Foon, (Monoclonal Antibody Therapy of Cancer: Preclinical Models and Investigations; Basic and Clinical Tumor Immunology, Vol. 2, Kluwer Academic Publishers, Hingham, MA) and Uhr (Journal of Immunology 133:i-vii, 1984). Interest in the potent higher plant toxin molecules peaked with the development of monoclonal antibodies because it appeared that the latter could be used as highly specific targeting agents for these toxins.
In general, the higher molecular weight toxins have characteristic A and B chains, with the B chain responsible for binding (usually via lectins to oligosaccharides) and A chains that act catalytically to irreversibly inhibit elongation factor 2 (EF2), therefore preventing protein synthesis. The vision was that the specificity of the antibody could substitute for the non-specific binding of B chain and deliver A chain selectively to tumor cells. More recently, a class of compounds called "ribosomal inactivating proteins" (RIPs) have been discovered that represent the equivalent of A chains without any associated B chain.
A number of obstacles emerged, however, that compromised the realization of this simple vision First, it was apparent that it was critical to develop systems to remove B chain from A chain beyond purity achieved with simple affinity chromatography. The RIPs and cloned toxins represent one practical solution to this problem. Second, the reticuloendothelial system removes macromolecules from the circulation, especially those that have been altered, such as an antibody that has been bound to toxin Third, it became apparent that there were receptors for the carbohydrates that exist naturally on the protein plant toxins. These also contributed to non-specific uptake and, therefore, toxicity. Finally, it became clear that B chain was critical for more than just binding to the cell, and seemed to facilitate the translocation of the A chain into the cell and eventually into the cytoplasm, where it effected its cytotoxicity.
Due to these obstacles, there is a need in the art for a class of conjugates that overcome the problems noted above, while concurrently possessing the capability of killing defined populations of cells, such as tumor cells, on a selective basis The present invention fulfills this need and further provides other related advantages.